Table of Content
Introduction to Sound
- Sound is a form of energy that travels in the form of vibrations through the air or any another medium.
- Vibration is defined as a rapid to and fro or up and down movement about a mean position.
- Vibrations travel through a medium such as air and reach our ears where they are interpreted by the brain as sound.
- Human beings are capable of producing sound which originates from the larynx or the voice box in the neck. The sound produced is controlled by thin membranous structures known as vocal cords that stretch across the larynx.
- The vibrating body can be- a stretched animal hide (as in drums and table), stretched strings (as in guitar and sitar), or air columns (as in flute or pipes).
You can produce slower vibration in a simple pendulum. It consists simply of a weight hanging by a thread. When the ball is given a small push, it performs to and fro movements which you can easily observe. These slow vibrations are also known as oscillations.
Characteristics of vibration
The important characteristics of vibrations are its frequency, amplitude and time period
. These determine the characteristics of the sound produced.
- The number of vibrations made by the vibrating body in one second is known as its frequency.
- The SI unit of frequency is the hertz (Hz).
The maximum extent of vibration of the vibrating body from its mean position is known as its amplitude.
The time taken by the vibrating body for one complete vibration is known as the time period of vibration. It is denoted by T.
T=1/f and f=1/T
Time period and frequency are reciprocals of each other.
Characteristics of sound
Sound can be characterized by the loudness, pitch and quality.
- Based on the amplitude of a sound wave, we can determine the loudness of the sound. When the amplitude is high, it will produce a sound that is loud and when the amplitude is low, it will produce a sound that is soft.
- Loudness is proportional to the square of the amplitude. This means that if the amplitude is doubled, the loudness increases four times.
- Pitch denotes the shrillness or flatness of a sound. Sound can be high or low.
- A woman’s voice generally has a high pitch than a man’s voice. This is because the frequency of a woman’s voice is higher.
The quality of a sound is that property by virtue of which two sounds of the same pitch and loudness produced by the two different musical instrument or people can be distinguished.
How sound travel?
- Sound travels through a medium in the form of waves. When vibrations take place, they are transmitted through a medium and form alternate compression and rarefactions.
- Compressions are regions in the medium where the particles are closer whereas rarefactions are regions in the medium where the particles are spread out.
Speed of sound:
The speed of sound changes with the change in medium. Speed also depends on the physical state and temperature of the medium. At higher temperature, the speed of sound is higher, while at lower temperature, the speed decreases.
The speed of sound is maximum in solids (5920m/s in steel), lesser in liquids (1480m/s in water) and minimum in gases (330m/s in air).
How does human hear sound?
Sound travels in the form of waves of vibrating air molecules. When these waves reach our ears we hear the sound.
The ear has three parts: Outer ear, Middle ear and Inner ear.
It consists of the pinna, ear canal, and the eardrum or the tympanum. The pinna gathers the sound waves and then leads to the ear canal from where they strike the eardrum.
The vibrations from the eardrum are transmitted to three closely-packed bones called the hammer, anvil and stirrup. These three bones finally transmit the vibrations to the inner ear.
This consists of cochlea, which is a long coiled tube. It contains a fluid and has numerous cells with hair. These sensitive hair cells transmit the vibrations to the brain through the auditory nerve, for the brain to register the sound.
Audible and inaudible sounds:
- Audible sounds are those that can be heard while inaudible sounds are those that cannot be heard.
- Human can hear sounds with frequency between 20Hz and 20,000Hz.
- Low frequency sounds which cannot be hear are called infrasonics.
- Objects that vibrate at frequencies of above 20,000Hz produce sound which also cannot be heard by us. Such sounds are called ultrasonics.
Reflection and absorption of sound:
Sound heard after reflection from a surface is called echo.
Just like heat or light, when sound falls on a surface, it is partly reflected and partly absorbed.
Soft surfaces are better absorbers of sound whereas hard surfaces are better reflectors of sound.
Uses of Echo:
- Echo is used in SONAR (Sound Navigation and Ranging) to find the depth of seas or distance of submarines.
- In concert halls, echo is desirable to some extent, because it enhances the sound and produces a pleasing effect.
- Bats use the principle of echolocation to avoid hitting against obstacles in their path.
Musical sounds and noise:
Sounds can be classified as musical sounds and noise.
The sounds produced by a tuning fork, violin, veena, flute and piano are pleasing to the ear. They are called musical sounds. They are produced by regular, periodic vibrations.
Certain sounds such as thunder, the rattling of wheels on a rough road, or a large number of people talking at the same time inside a room are unpleasant to hear. These sounds are called noise and produced by irregular and non-periodic vibrations.
Musical instruments are categorized into three types:
Stringed instruments, wind instruments and percussion instruments.
Stringed instruments make use of a string or wire to produce vibrations and sound. The frequency of sound is varied by varying the length of the vibrating wire.
In a sitar, the shorter the length of the wire, the higher the pitch it produces.
Wind instruments use the principle of a vibrating air column to produce sound. The frequency is varied by changing the length of the vibrating air column.
Flute, shehnai and clarinet are some well known wind instruments.
They are instruments in which vibrations of a stretched animal hide produce sound. The frequency of vibration can be increased by stretching the hide more.
Table, drums and mrindangam are some examples of percussion instruments.
Too much noise in our surroundings is known as noise pollution.
The loudness of sound is measured in decibels (dB).
Sources of noise pollution:
The sources of noise pollution include road traffic, jet planes, trains, construction sites, factories, uses of loudspeakers, lighting of crackers during festivals, and noise from radio and television.
Harmful effects of noise pollution:
- Sudden exposure to high noise level can cause permanent deafness by rupturing the eardrum.
- Noise environment causes headache and inability to concentrate on work.
- High noise levels can also lead to nervous tension and increase of blood pressure.
How to control noise pollution?
- Vehicles should be fitted with more effective silencers.
- Restricting the use of loudspeakers or amplifiers in public places.
- Using sound absorbing materials like curtains and rugs inside the home and planting trees along the roadside helps to reduce noise.
Practice questions for Sound
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1. How sound is produced?
2. What do you understand by a ‘wave’?
3. Write three differences between sound and light waves.
4. What do you understand by “sound energy cannot be produced”?
5. What is the name of the wave that can travel through vacuum?
6. Explain by some experiment that sound waves require medium for their propagation.
7. How sound waves travel through some medium?
8. Why sound waves do not propagate through vacuum?
9. What is the scientific name for the following? The number of vibrations made per second.
10. Why a sound cannot be heard on the moon?
11. Give two points of difference between longitudinal and transverse waves.
12. How will you prove that the sound waves exhibit longitudinal behaviour?
13. What are rarefaction and compression in case of sound waves?
14. Distinguish between crests and troughs.
15. Write the SI unit of velocity of a wave.
16. What are the factors that describe the sound wave and define them?
17 Why is a thundering sound heard later than lightening?
18. How RADAR is different from SONAR?
19. How far is a compression and its nearest rarefaction in a longitudinal wave?
20. Define sound ranging.
Class 8 Maths
Class 8 Science